The invention relates to a communications arrangement forming part of a communications system, in particular, but not exclusively, an SDH-DCN communications system.
A typical OSI (Open Systems Interconnection) routeing scheme involving the so-called “IS-IS Routeing Protocol” is illustrated in FIG. 1. In FIG. 1 a wide-area network (WAN) is shown divided into two domains, each domain being split into two areas. Each area contains a number of systems, which are designated either as end-systems (ESs) or intermediate systems (ISs). The ESs, which may represent hosts or various devices (e.g. servers), may be linked to one or more ISs via either point-to-point or broadcast circuits in a LAN (Local Area Network) or, for a geographically larger area, a MAN (Metropolitan Area Network) or WAN (Wide Area Network).
Routeing of message packets from any ES in one area to another ES in the same or another area is conventionally carried out under separate routeing protocols which correspond to a particular routeing hierarchy. Routeing between ESs and ISs is by way of the ES-IS protocol; that between any two ISs within the same area is via the intra-domain IS-IS protocol (Level 1), and that between two ISs in different areas is via the intra-domain IS-IS protocol (Level 2). Routeing between two different domains is outside the scope of the IS-IS protocol. However, the protocol provides a way to disseminate the inter-domain routeing information to all the inter-area routers, or level 2 Intermediate Systems, as they are called.
Details of the IS-IS intra-domain routeing protocol between intermediate systems are given in ISO/IEC Recommendation 10589, first revision (1992 Jun. 15) “Information technology—Telecommunications and information exchange between systems —Intermediate system to Intermediate system intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)”. ITU-T Recommendation G.784 June 1999) “General Aspects Of Digital Transmission Systems; Terminal Equipments” addresses management aspects of the SDH, including the control and monitoring functions relevant to SDH network elements.
Two types of routeing have traditionally been employed: static routeing and dynamic routeing. With static routeing, some Intermediate Systems in a domain store routeing criteria of various types. Such criteria are manually entered by the operator and are used to match the destination address of a packet against the criteria, to ascertain whether the packet may be routed on the circuit to which the static route is associated. With dynamic routeing, each system keeps a table containing the state of all routes within its scope. The table is updated on a continual basis. Since dynamic routeing is adaptive, being able to take account of broken links between systems or to take account of systems themselves being out-of-service, and is also decentralized, it has clear advantages over static routeing and is therefore the dominant form of routeing currently employed, at least under the intra-domain IS-IS protocol.
The ISs are divided into two main types: level 1 (L1), which routes packets within a particular area, and level 2 (L2), which routes packets between areas and between domains. Usually, an L2 IS also has an L1 routeing function, and is therefore actually an L1/L2 IS.
For dynamic routeing to occur, the following conditions must be satisfied.                Each IS must be apprised of the state of its neighbour ESs. In the same way, each ES must be apprised of the state of its neighbour ISs.        Each L1 IS must be apprised of the topology of the area of which it is part.        Each L2 IS must be apprised of the topology of the level 2 sub-domain of which it is part (that is, the partition of the domain made up of L2 ISs and of the links between them).In order to accomplish this, all the End and Intermediate Systems in the domain exchange “hello” packets, to know who are their neighbours. ISs also generate and flood LSPs (the LSP, or Link State Protocol data unit, is a packet containing the list of the neighbours of the originating IS), so that they become aware of the topology of the partition of the network within their scope (where the scope is the area, for level 1 ISs, and the level 2 sub-domain, for level 2 ISs). In this way, ISs are enabled to make the appropriate routeing decisions both at the L1 and at the L2 routeing level.        
The dynamic link-state updating process just described does not occur on the inter-domain level, however, but instead a static routeing method has to be employed in order to route packets from one domain to another. In order to achieve this, the routeing tables of the L2 ISs are provided with “reachable address prefixes” (RAPs), which are generated either manually, or by means of a dynamic inter-domain routeing protocol. Such RAPs provide routeing criteria for the packets that may not be routed on the basis of the dynamic routeing information available (as they are addressed outside the domain). The criterion is that if the destination address of the packet begins with a pattern matching an existing prefix, it may be forwarded on the circuit associated with such a prefix (which circuit will turn out to be a domain boundary).
In an actual routeing exercise, an L1 IS will receive a packet from one of its associated ESs (note that if a system acts both as an End and as an Intermediate System, this is represented by the IS having itself as an ES neighbour). If the packet is destined for an ES in the same area, it will be routed by that L1 IS either to the destination ES directly, or via one or more other L1 ISs. If the packet is destined for an ES outside the source area, the L1 IS will pass the packet on to the nearest L2 (or L1/L2) IS in the source area (possibly passing through one or more other ISs). Once the packet gets to the L2 sub-domain, it will be passed on to an appropriate L2 (or L1/L2) IS in the destination area. Finally, the packet is delivered by L1 routeing to the destination ES, either directly, or via one or more other L1 ISs.
In one particular type of telecommunications system, namely the SDH (Synchronous Digital Hierarchy) system, a ring arrangement of systems (called “network elements” in SDH terminology) is often employed. This is illustrated in FIG. 2, in which an Ethernet LAN 2 is connected to a ring 10 of network elements (NEs) A, B, C and D (only four are shown for the sake of simplicity), one of which—NE A—is designated as the “gateway NE” (GNE). The GNE (also known as the “head of the ring”) is the NE which provides access to the other NEs in the ring for the Element Manager (EM). The EM is a system (normally running on a computer) which performs administrative operation on SDH NEs, such as configuration, alarm and performance data management. The DCN (Digital Communications Network) is the network that provides the support for the dialogue between the EM and the NEs. In practice, there may be tens of NEs on a ring, and many tens of rings connected to a single GNE. As most of these NEs need to be functioning as ISs (because they have to route packets towards the further NEs), usually all the NEs are configured to act as ISs. Also, in practice there may be a number of GNEs present on LAN 2 (FIG. 2 shows a second ring 20 with its own GNE, GNE 2), and some of the NEs in a ring may in turn have their own sub-rings.
Each NE in each ring is equipped with a couple of data communication channels (DCCs) through which it communicates with the next and the previous NE in the ring. These DCCs are shown in FIG. 2 as channels “1” and “2” associated with NEs A, B, C and D.
The element manager 11, which is connected to another LAN, LAN 1, communicates with the rings 10 and 20 via the Data Communications Network (DCN) 12, via a router 13 upstream of the DCN, and via a router 14 downstream of the DCN and connected to the LAN 2. The routers are effectively ISs.
In the normal configuration, in which IS-IS dynamic routing (described earlier) is employed, router 14, all the GNEs on LAN 2 and all the NEs reachable through these GNEs (including NEs B, C and D) are located in the same IS-IS area. Since there may result a large number of ISs in that area, problems in routeing may be caused due to the restrictions in the number of ISs which the IS-IS protocol, by its design, can handle. In practice, the protocol suggests that a typical maximum configuration domain will contain at most 400 L2 ISs and at most 100 L1 ISs per area, while the domain is allowed to comprise up to 4000 systems.
The above-mentioned restrictions are due to the fact that each of the NEs in the area has, in the conventional arrangement, a complete view of the topology of the area (as explained earlier). Thus, an NE reachable through a GNE on one ring (e.g. GNE 1) has to process all the ISPs generated by any other NE on any ring in the same area (e.g. one of the NEs in ring 20), and this can lead to various problems, such as memory exhaustion, CPU overload and traffic bursts due to the routeing messages (such bursts may be particularly critical when there is a sudden change in the network topology).